Recently, a lot of vehicles include a tire air pressure detecting device e.g., US 2004/0055370A1(JP 2003-528378A) for giving a warning on detecting decrease of a tire air pressure. In general, the tire air pressure detecting device turns on a warning light provided at an instrument panel in a passenger compartment in giving the warning. In this case, a value indicating the tire air pressure may also be displayed.
However, a driver knows little about implications of the tire air pressure. For instance, it is hardly known that the tire air pressure decreases naturally and gradually, specifically by about 20 kPa per month. It is hardly known as well that decrease of the tire air pressure has negative influence on drivability, fuel efficiency, and so on.
Therefore, the driver tends to ignore the warning given by the tire air pressure detecting device because of lack of knowledge about the negative influence. The driver tends to think that the warning is a consequence of malfunction of the tire air pressure detecting device because he/she understands erroneously that the warning light is turned on while a tire in concern is not in a bad condition.